The present invention relates to a device and a method for counting elementary particles (such as α-particles, γ-particles, electrons or positrons) emitted by a fluid, the device having a line for transferring this fluid and, outside the line, means for detecting these particles that are attenuated by a wall of the line and/or by this fluid. The invention applies more particularly, but not exclusively, to a fluid consisting of blood microsamples taken from a mammal.
As is known, the counting of particles attenuated by certain components of a fluid that incorporates them and/or by the wall of the line transferring this fluid poses certain technical problems, which are particularly acute in the particular case of counting within small fluid samples, in the case of absorption of these particles by the fluid, or else in the case when the particles to be counted are electrons or positrons emitted by beta-radioactivity in particular. Among these applications in which these three problems are combined, mention may mainly be made of those for direct measurement of the positrons or electrons coming from the beta-radioactivity contained in blood microsamples, for example for devices for measuring the entry function of a mammal intended for quantitative imaging.
A number of laboratories, such as that at Sherbrooke in Canada, have developed positron or electron detectors for blood microsamples incorporating beta-emitters as radiotracers, by providing counting devices, the detector efficiency of which does not exceed 7% in the case of 18F radiotracers and 16% in the case of 11C radiotracers. These devices include a tubular line of constant flow section over its entire length, without any particular feature in its detection zone provided with the detectors. This line has an internal diameter of 0.58 mm and since the mean free path of positrons in blood is about 0.6 mm in the case of 18F, it follows that few positrons emerge from these blood microsamples. In addition, the material used for this line—a low-density polyethylene (LDPE)—since it has a very large wall thickness e (e≧150 μm and typically equal to 192.5 μm) for a density d close to 1 g/cm3 (typically equal to 0.92 g/cm3 in the case of “PE10”), it follows that this line results in a relatively high attenuation of the electrons or positrons, this being proportional, as is known, to the product e×d, which here is typically equal to 192.5×0.92, i.e. about 177.
Also known, by the name “β-microprobe”, are other particle counting methods and devices that are used by the French company Biospace for measuring the entry function of a small mammal, such as a rat or a mouse. These methods essentially consist in eliminating the aforementioned line, introducing the detector directly into an artery of the animal, in the state immersed in the blood of the latter.
One major drawback of this lineless solution is that the radiotracer accumulated in the other organs of the animal creates a strong perturbation by gamma-radiation. Another drawback of this solution lies in the small dimensions characterizing the measurement, which leads to a low counting efficiency. It is therefore necessary to add a second detector to the main detector, the second detector being placed in the body of the animal but not in its blood, and a differential procedure is used to subtract the background noise due to this gamma-radiation.
Even under these measurement conditions, it turns out that the stated detection efficiency (which is reduced by absorption in the blood) is only 16% and 20% in the case of radiotracers based on 18F and 11C, respectively, using a scintillating plastic detector 1 mm in diameter.
Given the recent developments in technologies for treating microsamples and the desire of biologists to develop tools intended for small animals, such as mice, it is of fundamental importance to be able to measure the activity of liquid microsamples containing the least possible amount of radiotracers. Since these microsamples have a volume of the order of a few μl (typically 8 to 30 μl), it is necessary for the counting devices used to have a high efficiency (i.e. a high measurement sensitivity), particularly for studying short-lived radiotracers such as 11C, the activity of which at the end of a sequence decreases significantly (the half-life of 11C being 20 minutes).